The invention relates to a fluorescent lamp comprising a glass discharge vessel in which a gas is present, which discharge vessel is provided on either side with a tubular end portion with a longitudinal axis, each end portion being provided with a glass stem, while an exhaust tube extends axially outwards from the stem for receiving and/or discharging gases during lamp production, and an electrode extends through the stem axially inwards for maintaining a discharge in the discharge vessel.
An example of such a fluorescent lamp is one in which the end of the electrode extending in a inward direction is surrounded in a radial direction by a shield for catching material emitted by the electrode, which shield is fastened on an elongated support wich from the stem in an inward direction.
Mercury is the primary component for the (efficient) generation of ultraviolet (UV) light in mercury vapor discharge lamps. A luminescent layer comprising a luminescent material (for example a fluorescent powder) is present on the inner wall of the discharge vessel for the conversion of UV to other wavelengths, for example to UV-A and UV-B for sun tanning purposes (solarium lamps), or to visible radiation for general lighting purposes. The discharge vessel of a fluorescent lamp usually has a circular cross-section and comprises elongate embodiments (TL tubes) as well as compact embodiments (power-saving lamps). In the TL tube, said tubular end portions lie in one another""s extensions and form a long, straight tube, whereas the end portions are interconnected via a curved tubular portion or a so-called bridge in the case of a compact power-saving lamp.
The fluorescent lamp is evacuated during manufacture through the glass exhaust tubes which are present at both ends of the lamp. The desired gas mixture is subsequently introduced into the lamp through these same exhaust tubes, whereupon the exhaust tubes are pinched shut and sealed off.
During operation, a voltage is maintained between the electrodes, which are also present at both ends of the lamp, so that a continuous discharge takes place and the mercury vapor emits said UV light. The ends of the electrodes are radially surrounded by respective shields, i so desired, because small particles are regularly emitted by the electrodes during operation, which particles would end up on the inner wall of the discharge vessel. This is undesirable because it reduces the light output in the relevant location, and the lamp would show al irregular light output, which is why said particles are intercepted by the shield. The shield, if present, is fastened in the glass stem by means of a wire-type support.
A problem which may arise in such a fluorescent lamp is that towards the end of lamp life, when the electrodes have been partly exhausted, the discharge may continue between portions of the electrodes which were not designed for this purpose, with the result that the stem will be covered with metal particles originating from these portions of the electrodes. A shield if indeed present, only protects in radial directions. As a result, the outer surface of the stem becomes conducting, so that the discharge will attach itself thereon, and the stem becomes so hot that it softens and deforms. A further consequence is that an unfavorable heat distribution causes the discharge vessel wall to become excessively hot for a longer period. It may occur in the end that the glass discharge vessel is cracked by the heat.
It is an object of the invention to provide a reliable fluorescent lamp in which the risk of the discharge vessel wall becoming hot at the end of lamp life is counteracted in a simple and efficient manner.
According to the invention, a fluorescent lamp of the kind mentioned in the opening paragraph s for this purpose characterized in that the fluorescent lamp complies with at least one of he following equations:       ξ    =                                                      R              1                                      R              2                                +          1                                                    R              3                                      R              4                                +          1                     less than               0.4        ⁢                  xe2x80x83                ⁢                  and          /          or                          α    =                            R          1                          R          2                     less than       0.25      
in which
R1/R2=a measure for the effectivity with which the thin glass surface of the stem where the exhaust tube is fused to the stem (the so-called weak spot) is heated;
R3/R4=a measure for the effectivity with which heat is transported from the stem to the discharge vessel by means of radiation.
As will be explained in more detail below with reference to the Figure, the invention is based on the recognition that an existing lamp whose construction complies with one of the equations given above will exhibit a passive behavior towards the end of its useful life, i.e. the thin glass surface of the stem in the location where it is fused to the exhaust tube (the so-called weak spot) is quickly heated up so as to create a leak there, which will extinguish the lamp in time, i.e. before the glass discharge vessel as a whole is heated up too much.
A preferred embodiment of the fluorescent lamp according to the invention is characterized in that the current supply wires of the electrode are coated with a material which has a better thermal conductivity than nickel, which material preferably comprises copper. In another preferred embodiment, the current supply wires are entirely made of this material.
A further preferred embodiment of a fluorescent lamp according to the invention is characterized in that the distance from the core of at least one of the current supply wires of the electrode (preferably both current supply wires of the electrode) to the outer shell of the exhaust tube is smaller than 0.7 mm, in particular smaller than 0.4 mm, more in particular smaller than 0.2 mm.
A further preferred embodiment of a fluorescent lamp according to the invention is characterized in that the arithmetical product of the wall material thickness of the exhaust tube and the diameter thereof is smaller than 3, in particular smaller than 2, more in particular smaller than 1 mm2 for a lamp having a discharge vessel diameter greater than 2.54 cm (for example lamp types T8 and T12), and smaller than 1.5, in particular smaller than 1, more in particular smaller than 0.5 mm2 for a lamp having a discharge vessel diameter smaller than 2.54 cm (for example, lamp types PL, T5, and CFL).
A further preferred embodiment of a fluorescent lamp according to the invention is characterized in that the arithmetical product of the wall material thickness of the end of the discharge vessel (the carrier) and the diameter thereof is smaller than 9, in particular smaller than 8, more in particular smaller than 7 mm2 for a lamp having a discharge vessel diameter greater than 2.54 cm (for example, lamp types T8 and T12), and smaller than 4, in particular smaller than 3, more in particular smaller than 2 mm2 for a lamp having a discharge vessel diameter smaller than 2.54 cm (for example lamp types PL, T5, and CFL).
The invention also relates to a method of manufacturing a fluorescent lamp whereby a glass discharge vessel is provided with a tubular end portion with a longitudinal axis at either end, said end portion being provided with a glass stem, and an electrode is arranged in axial inward direction through the stem for maintaining a discharge in the discharge vessel, and an exhaust tube is provided so as to extend axially outward from the stem, through which exhaust tube the discharge vessel is filled with a gas, characterized in that at least one of the following equations is complied with:       ξ    =                                                      R              1                                      R              2                                +          1                                                    R              3                                      R              4                                +          1                     less than               0.4        ⁢                  xe2x80x83                ⁢                  and          /          or                          α    =                            R          1                          R          2                     less than       0.25      